If you have used Firefox in Windows 7 or, if one has used Google Chrome
Web browser, you shall know, what exactly I mean by having 'tabs-in-titlebar' , which incidently is not available in linux platform either by
default or via an addon.
Thank you.

On Linux the Firefox button is on the Tab bar and you usually have the title bar visible via the OS setup.
It is not possible to have the tab bar in the title bar on Linux or place other items on the title bar.
Only Firefox for Windows has that feature and only if the Firefox window is maximized you can have the Firefox button and the tab bar in the title bar.
On Windows the title bar is in the Firefox DOM (#titlebar), so it is possible to place other items on the title bar.